The effects of global warming are making it more difficult for reservoir managers to control floods and manage flows for irrigation, recreation and fisheries.

Two days of record high temperatures and two days of record rainfall the same week in late April sent 26,000 cubic feet per second surging into the Boise River dam system, forcing federal river managers to increase flows to more than 8,100 cfs — the highest flow out of Lucky Peak Dam since 1998 and just the second time it has hit 8,100 in 30 years.

“If the reservoir had been full, we would have had a big problem,” said Patrick McGrane, manager of river operations for the Bureau of Reclamation’s Pacific Northwest Region, which operates the Boise River reservoirs in partnership with the U.S. Army Corps of Engineers.

As late as the middle of January, this looked as if it was going to be a dry year across southern Idaho, especially in the Boise Basin, where Bogus Basin ski area had its latest opening in history.

But then the snows finally came. And in March, much of the precipitation fell as rain, causing the Payette and Weiser rivers to threaten flooding, said Ron Abramovich, a water-supply specialist with the Natural Resources Conservation Service in Boise.

The April warm spell and rains are examples of the higher variability that experts such as Abramovich say we can expect because of global warming. That’s making it harder to predict how reservoirs will fill — and what the flows will be in rivers with and without dams.